MHC class I genes encoding transplantation antigens are ubiquitously expressed, although their level of expression varies among tissues. Analysis of the 5' flanking DNA sequence of a swine class I gene has demonstrated that this region contains a series of negative and positive regulatory elements. One of these elements, consisting of overlapping negative and positive regulatory elements, is a regulatory domain responsible for establishing tissue-specific levels of MHC class I gene expression. Introduction into transgenic mice of a series of nested deletion mutants which differ in the extent of the regulatory domain, reveals that the enhancer activity predominates in lymphoid tissues. The tissue-specific domain forms distinct enhancer and silencer associated complexes with cellular trans acting factors. Enhancer binding activity is present in extracts from various cell types, independent of levels of class I expression. In contrast, the level of silencer binding is inversely proportional to the level of class I gene expression. Thus, class I genes are found to be negatively regulated. Biochemical characterization of the regulatory factors has demonstrated that each factor consists of at least two distinct components, one of which appears to be common to both factors. Both the silencer and enhancer factors are redox-sensitive. The enhancer factor complex is approximately 3OkD. The enhancer unique subunit is also glycosylated. The silencer factor complex is approximately 95kD. An additional negative regulatory element has been identified which the proto-oncogene, c-jun, binds as a homo or heterodimer, and acts as a specific negative regulator. These observations. suggest that class I expression is actively regulated during cell activation, and also suggest a mechanism whereby cells transformed by oncogenic variants of c-jun could decrease class I expression.